Lignin

Lignin, as a natural polymer, exists in all terrestrial plants, although some aquatic organisms may contain lignin or “lignin-like” components. Lignin molecules, starting with dimerization of two monolignol radicals, grow via cross coupling between a monolignol radical and the previously formed dimeric or oligomeric lignol radicals. Lignins are plant polymers made from phenylpropanoid building units. They contain most of the wood methoxyl content. Lignins are resistant to acid hydrolysis, readily oxidized, soluble in hot alkaline and bisulfite, and readily condensed with phenols or thiols. Many attempts have been made to define lignin or lignins based on the constitution, structural features, and mechanism of formation. The problem of lack of precise definition for lignin is associated with its nature: no regularly repeating multi-unit structures have been found, and compositions and structures of lignin vary depending on their origins. Any simplified definition would risk excluding aspects of lignin. Lignins in grass plant cell walls are much more complex than those in wood. Lignin precursors, monolignols, are more diverse in grasses, and the biosynthetic pathways for monolignols are still not quite clear as that for monolignols in woods and dicots. Because of the intimate associations between polysaccharides and lignins in grasses, better procedures for isolating lignins from grass plant cell walls are still needed to better understand structures of grass lignins, as well as their cross-linking to polysaccharides. In grasses, cross-link polysaccharides and lignins form so-called lignin–hydroxycinnamate–polysaccharide complex, which severely decreases the digestibility of cell wall polysaccharides by ruminants.